QUANTUM DYNAMICS OF OVERTONE RELAXATION IN BENZENE .2. 16 MODE MODEL FOR RELAXATION FROM CH(V=3)

This series is concerned with the quantum dynamics of overtone relaxation in benzene and in reduced mode benzene fragments. In part II of this series, emphasis is placed upon time-dependent studies of v = 3 CH overtone relaxation in a 16 mode planar benzene model (5 CH stretch modes are inactive out of the 21 planar modes). Through use of the wave operator contraction algorithm, primitive vibrational basis sets containing up to 1.1 X 10(9) states are contracted to active spaces of dimension about 4000 states. The exact dynamics within the active space is developed with the recursive residue generation method. The time-dependent survival probability of the CH(v = 3) overtone is compared with results from classical mechanics and with the Fourier transform of the experimental overtone spectra. The classical and quantum results are in good agreement only at short times (t < 0.1 ps). At longer times multiple small recurrences are present in the quantum results. The quantum and experimental results share common features (rapid falloff at short times, followed by multiple small recurrences), but there are quantitative differences. The sensitivity of the quantum survival to alterations in the anharmonic potential is also studied. The short-time dynamics (t < 0.5 ps) is shown to be accurately reproduced by reduced 4 and 5 mode models. Only a small subset of states in these carefully selected modes are significantly populated during the early decay of the v = 3 overtone, even though the total density of states is very large.